Layer-by-layer programming of built-in strain in Co bilayers

We demonstrate layer-by-layer programming of built-in strain in sputter-deposited Co bilayers on flexible substrates. Conventional strain engineering, whether epitaxial or based on post-deposition actuation, does not readily allow the strain state to be set independently for each layer of a multilayer. Here, we use the substrate tension during each deposition step as a layer-specific writing parameter, sputter-depositing two Co layers separated by a 3 nm-thick Pt spacer under independently chosen uniaxial tensions. This yields an asymmetrically strained bilayer, in which only one Co layer carries built-in strain, and a cross-strained bilayer, in which the two Co layers carry mutually orthogonal built-in strains. Magnetization measurements show that each Co layer exhibits the magnetic anisotropy expected from its programmed strain state. This layer-specific response is preserved even though the two Co layers are separated by only a few nanometers, providing magnetic evidence for an abrupt change in the effective built-in strain state across the spacer, which we interpret as a nanoscale elastic discontinuity that would be difficult to realize in a coherent epitaxial strain field. These results establish deposition-defined built-in strain as a depth-selective design variable for thin-film heterostructures.